Closed Loop Laser Control

ABSTRACT

The radiance of a laser is a function of drive current. The radiance is also a function of other factors, such as age and temperature. A laser projection device adjusts laser drive parameters using a gradient descent operation. The device parameters may be adjusted iteratively and periodically. The period may be shorter or longer than a scan line in a video image.

FIELD

The present invention relates generally to laser projection systems, andmore specifically to laser control in laser projection systems.

BACKGROUND

Lasers emit light in response to a current. For example, laser diodesproduce light in response to a current in the diode. Also for example,semiconductor lasers also produce light in response to a current. Theoutput radiance of a laser varies as the drive current is varied. Theoutput radiance of a laser may also vary due to other factors. Forexample, the output radiance of a laser diode may vary with age. Alsofor example, the output radiance of a laser diode may vary as thetemperature of the diode varies. This can be problematic in part becausethe temperature of a laser diode may be affected by ambient temperaturechanges as well as the historical drive current which results in“self-heating” of the diode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a laser projection apparatus with closed loop lasercontrol;

FIGS. 2A-2C show various laser characteristics and how they may becompensated;

FIGS. 3 and 4 show micro-projectors;

FIG. 5 shows a laser projection apparatus with closed loop lasercontrol;

FIG. 6 shows a color laser projection apparatus with closed loop lasercontrol;

FIG. 7 shows a flowchart in accordance with various embodiments of thepresent invention; and

FIG. 8 shows a mobile device in accordance with various embodiments ofthe present invention.

DESCRIPTION OF EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention. It is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described herein in connection with one embodiment may beimplemented within other embodiments without departing from the scope ofthe invention. In addition, it is to be understood that the location orarrangement of individual elements within each disclosed embodiment maybe modified without departing from the scope of the invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims, appropriately interpreted, along with the fullrange of equivalents to which the claims are entitled. In the drawings,like numerals refer to the same or similar functionality throughout theseveral views.

FIG. 1 shows a laser projection apparatus with closed loop lasercontrol. Apparatus 100 includes image processing circuitry 130, radianceto drive current mapping 110, laser projector 120 and control circuit150.

In operation, laser projector 120 scans a light beam back and forthhorizontally and up and down vertically to “paint” an image. Theradiance of the light is changed at discrete points to illuminate pixelson a display surface. Image processing circuitry 130 determines theradiance of one more colors to be displayed at a pixel location in animage. Circuitry 130 presents a “commanded radiance” on node 102 tomapping component 110. Mapping component 110 maps the commanded radianceto a drive current value on node 104. Laser projector 120 receives thedrive current value on node 104 and projects light at 140. In someembodiments, laser projector 120 receives a variable current on node 104and this current is passed through a lasing device such as a laser diodeor semiconductor laser. In other embodiments, laser projector 120receives digital data on node 104 and laser projector 120 includes adigital-to-analog converter (DAC) to produce the current to pass throughthe lasing devices.

Laser projector 120 may include lasing devices that heat up or cool offbased on ambient temperature changes. Further, lasing devices withinprojector 120 may heat up when emitting light. Further, multiplehistorical drive currents may have a cumulative heating effect on thelasing devices. As the temperature of the lasing device changes, theradiance for a given drive current also changes. This can result invariations in overall radiance (brightness) of the displayed image.Other factors may also affect lasing device radiance over time. Forexample, the radiance of lasing devices may change over time due toaging.

Some lasing devices may be more sensitive than others to temperaturevariations. For example, a lasing device that emits green light may bemore sensitive to temperature variations than other color devices, suchas red and blue laser diodes. When one lasing device is more sensitivethan others, the color balance of the displayed image may change as afunction of temperature.

Control circuit 150 accounts for effects of temperature variations andother factors and modifies the mapping of commanded radiance to drivecurrent values within mapping component 110. In some embodiments, themapping is modified to correct for radiance variations only. In otherembodiments, the mapping is modified to correct for radiance as well ascolor balance.

Control circuit 150 provides three parameters: alpha, beta, and gamma,(α, β, γ) to mapping component 110. Mapping component 110 scales theradiance data using a radiance gain value a, scales the current using adrive current gain value β, and provides an offset to the current usinga drive current offset value γ, as follows:

I=β×f(αR)+γ  (1)

where R is the commanded radiance, I is the resulting current, and f(x)is a nominal R-I mapping that represents a nominal inverse lasercharacteristic. Control circuit 150 receives the commanded radiancevalues from image processing circuit 130, and also receives measuredradiance values from laser projector 120. Control circuit 150 comparesthe commanded radiance to the measured radiance and modifies one or moreof (α, β, γ) to correct for radiance and/or color balance variations.

Values for (α, β, γ) may be iteratively adjusted. An iteration may haveany period. For example, in some embodiments, (α, β, γ) are updated foreach video line in laser projector 120. Also for example, in someembodiments, (α, β, γ) are updated multiple times within each videoline. The parameters (α, β, γ) may be abruptly modified at eachiteration or may be slewed using small steps between iterations.

In some embodiments, control circuit 150 performs a “steepest descent”or “gradient descent” operation to determine values for (α, β, γ) ateach iteration. Gradient descent is based on the observation that if thereal-valued function F(x) is defined and differentiable in aneighborhood of a point a, then F(x) decreases fastest if one goes froma in the direction of the negative gradient of F at α, −∇F(a), where ∇is the gradient operator. It follows that, if b=a−Γ∇F(a) for Γ>0 a smallenough number, then F(a)≧F(b). With this observation in mind, one startswith a guess x₀ for a local minimum of F, and considers the sequence x₀,x₁, x₂, . . . such that x_(n+1)=x_(n)−Γ∇F(x_(n)), n≧0. This yieldsF(x₀)≧F(x₁)≧F(x₂) . . . such that the value of F decreases at eachiteration.

In various embodiments of the present invention, a cost function derivedfrom the error between commanded radiances and measured radiances isused for the function F of the previous paragraphs. For example, thefollowing gradient descent equations may be used to iterate values for(α, β, γ) using cost functions J:

α_(n+1)=α_(n) −Γ∇J _(α)(α_(a),β_(n),γ_(n))   (2)

β_(n+1)=β_(n) −Γ∇J _(β)(α_(n),β_(n),γ_(n))   (3)

γ_(n+1)=γ_(n) −Γ∇J _(γ)(α_(n),β_(n),γ_(n))   (4)

The cost function J may be any function that provides a metric relatedto the measured radiance. For example, suitable cost functions mayinclude, but are not limited to, an error function, a squared errorfunction, absolute value of an error function, normalized errorfunction, or any combination. For specificity, and not by way oflimitation, the remainder of this description treats the cost function Jas a squared error

J=ε ²(α,β,γ),   (5)

and the gradients with respect to the variables (α, β, γ) are thepartial differentials of the cost function with respect to thosevariables, or:

$\begin{matrix}{{\nabla J_{\alpha}} = {2ɛ\frac{\partial ɛ}{\partial\alpha}}} & (6) \\{{\nabla J_{\beta}} = {2ɛ\frac{\partial ɛ}{\partial\beta}}} & (7) \\{{\nabla J_{\gamma}} = {2ɛ{\frac{\partial ɛ}{\partial\gamma}.}}} & (8)\end{matrix}$

In some embodiments, the error function may be the difference betweenthe commanded radiance and the measured radiance, or:

ε=R _(C) −R _(m),   (9)

and the commanded radiance can be represented as a function of (α, β, γ)and the inverse of the mapping function as:

$\begin{matrix}{{R_{C} = {\frac{1}{\alpha}{f^{- 1}\left\lbrack \frac{I_{C} - \gamma}{\beta} \right\rbrack}}},} & (10)\end{matrix}$

where f is the nominal R-I mapping function shown inside 110 in FIG. 1.

The partial differentials of the error function become:

$\begin{matrix}{{\frac{\partial ɛ}{\partial\alpha} = {\frac{\partial R_{C}}{\partial\alpha} = {{\frac{- 1}{\alpha^{2}}{f^{- 1}\left\lbrack \frac{I_{C} - \lambda}{\beta} \right\rbrack}} = \frac{- R_{C}}{\alpha}}}},} & (11) \\\begin{matrix}{\frac{\partial ɛ}{\partial\alpha} = \frac{\partial R_{C}}{\partial\beta}} \\{= \left. {\frac{- 1}{\alpha}\frac{\partial R_{C}}{\partial I}} \middle| {}_{\frac{I_{c} - \gamma}{\beta}}\left\lbrack \frac{I_{C} - \lambda}{\beta^{2}} \right\rbrack \right.} \\{{= \left. {\frac{- 1}{\alpha\beta}\frac{\partial R_{C}}{\partial I}} \middle| {}_{\frac{I_{C} - \gamma}{\beta}}{f\left( {\alpha \; R_{C}} \right)} \right.},{and}}\end{matrix} & (12) \\\begin{matrix}{\frac{\partial ɛ}{\partial\gamma} = \frac{\partial R_{C}}{\partial\gamma}} \\{= \left. {\frac{1}{\alpha}\frac{\partial R_{C}}{\partial I}} \middle| {}_{\frac{I_{C} - \gamma}{\beta}}\left\lbrack \frac{- 1}{\beta} \right\rbrack \right.} \\{= \left. {\frac{- 1}{\alpha\beta}\frac{\partial R_{C}}{\partial I}} \middle| {}_{\frac{I_{C} - \gamma}{\beta}}. \right.}\end{matrix} & (13)\end{matrix}$

Various embodiments of control circuit 150 evaluate eqs. (11)-(13) whendetermining next values for (α, β, γ) at each iteration. Someembodiments of the present invention use fewer than all of (α, β, γ). Inthese embodiments only a subset of eqs. (11)-(13) may be evaluatedduring each iteration.

In some embodiments, parameters other than (α, β, γ) are employed. Forexample, referring back to eq. (1), a different parametric formulationfor the radiance to current mapping may be employed, and differentparameters (including more or less than three) may be adjusted at eachiteration. A fourth parameter may be added to eq. (1) by squaring thecurrent before or after the offset. Further, a general polynomialformulation may be used such as I=δ₃f³+δ₂f²+δ₁f+δ₀ where f is thenominal mapping and δ₃₋₀ are parameters that may be optimized using anyof the methods described herein.

The above description focuses on embodiments that utilize steepestdescent or gradient descent operations to adjust parameters, althoughthis is not a limitation of the present invention. Other iterativeoperations may be employed without departing from the scope of thepresent invention. For example, and not by way of limitation, variousembodiments may employ conjugate gradient methods, Newton's method,Quasi-Newton's method, or other suitable methods.

Control circuit 150 may have hardware or a combination of hardware andsoftware configured to perform the functions described. For example, insome embodiments, an application specific integrated circuit (ASIC)performs substantially all of the above computations in hardwaremultiple times for each video line. Also for example, in someembodiments, a processor may perform the above computations in softwareonce for each video frame. The various embodiments of the invention arenot limited to a hardware implementation, a software implementation, orany particular combination.

FIGS. 2A-2C shows various laser characteristics and how they may becompensated. FIG. 2A shows that the radiance gain value a can compensatefor changes in slope, but not changes in the threshold 210. FIG. 2Bshows that the drive current gain value β can compensate for bothchanges in slope as well as changes in threshold, while FIG. 2C showsthat the drive current offset value y can compensate for threshold. Asshown in FIGS. 2A-2C, changes in laser characteristics can becompensated for by modifying one or more of (α, β, γ).

FIG. 3 shows a micro-projector. Projector 300 may be used in apparatus100 (FIG. 1) as laser projector 120, although this is not a limitationof the present invention.

Projector 300 includes lasing devices 302, 304, and 306. Lasing devices302, 304, and 306 may be any type or combination of laser light emittingdevices, including but not limited to vertical extended cavity surfaceemitting lasers (VECSEL), distributed Bragg reflector (DBR) lasers,diode lasers, or any other. Projector 300 also includes mirrors 303,305, and 307, filter/polarizer 310, and micro-electronic machine (MEMS)device 318 having mirror 320. The lasing devices are driven by red,green, and blue radiance data (current) as described with reference toFIG. 1. Red, green, and blue light is provided by the lasing devices.Lasers typically produce light as a column, and this column emerges as anarrow beam. When each beam is directed at the MEMS mirror (eitherdirectly or through guiding optics) the colors of light can be mixed onthe surface of the mirror, pixel by pixel.

The MEMS mirror rotates on two axes to sweep the light beams in bothhorizontal and vertical directions. The trajectory that the beam takesis a function of the signals received from the sweep drive. In someembodiments, the beam may sweep back and forth horizontally in asinusoidal pattern. Further, in some embodiments, the beam may sweep upand down vertically in a sinusoidal pattern. In general, the beam may beswept in any combination of horizontal and vertical patterns, includinglinear and non-linear patterns. Pixels may be displayed when the beam issweeping in one direction or in both directions. For example, in someembodiments, pixels may be displayed as the beam sweeps down in thevertical direction, but not when the beam sweeps back up. Also forexample, in some embodiments, pixels may be displayed as the beam sweepsdown as well as when the beam sweeps up in the vertical direction.

This process of picture-building can be repeated many times per second,to reproduce moving pictures. Therefore, a MEMS mirror and three coloredlight sources can function like a traditional CRT monitor or televisionset, but without the metal and glass vacuum tube, and without thephosphors on a screen. Instead, this produces a small projector, with anearly infinite focal point.

By using solid-state colored continuous beam lasing devices, it ispossible to build such a projection device on the millimeter scale.Further, by modulating the power to each lasing device as needed toproduce a particular color, it is possible to greatly reduce theelectrical requirements of such a device. Together, this yields aprojection device that can fit into a small form factor device, and thatcan run reliably on its stored battery power. The MEMS based projectoris described as an example, and the various embodiments of the inventionare not so limited. For example, other projector types may be includedin projection systems with laser control as described herein withoutdeparting from the scope of the present invention.

Projector 300 also includes photodetectors (PD) 340, 350, and 360.Photodetectors 340, 350, and 360 are aligned such that each one detectslight of a different color passing through one of mirrors 303, 305, and307. Mirrors 303, 305, and 307 allow a predetermined amount of light topass through to the photodetectors while reflecting the remaining lightto mirror 3 10. The photodetectors supply measured radiance values to aclosed loop control circuit at 370.

Although photodetectors are shown detecting light on the back sides ofmirrors 303, 305, and 307, this is not a limitation of the presentinvention. In various embodiments of the present invention,photodetectors are placed in various locations to detect the differentcolors of light.

Photodetectors 340, 350, and 360 supply measured radiance R_(M) data toone or more feedback circuit(s) such as control circuit 150 (FIG. 1). Inresponse to the measured radiance data, control circuit 150 may thenadjust parameters (α, β, γ) for each lasing device separately using agradient descent operation as described above.

FIG. 4 shows a micro-projector. Projector 400 may be used in apparatus100 (FIG. 1) as laser projector 120, although this is not a limitationof the present invention.

Projector 400 includes lasing devices 302, 304, and 306, mirrors 303,305, and 307, filter/polarizer 310, and micro-electronic machine (MEMS)device 318 having mirror 320, all of which are described above withreference to FIG. 3. Projector 400 also includes integratedphotodetector 460 situated on the back side of filter/polarizer 310.Photodetector 460 detects light for all three colors and providesmeasured radiance data for all three lasing devices at 470.

Although integrated photodetector 460 is shown situated on the backsideof filter/polarizer 310, this is not a limitation of the presentinvention. For example, photodetector 460 may be situated anywheresuitable to measure the radiance of multiple lasing devices.

In some embodiments, integrated photodetector 460 measures radiance ofone or more lasers separately. For example, photodetector 460 maymeasure green light and then send measured radiance R_(M) for the greenlight back to control circuit 150 (FIG. 1). In response to the measuredradiance data, control circuit 150 may then adjust parameters (α, β, γ)for the green laser using a gradient descent operation as describedabove.

FIG. 5 shows a laser projection apparatus with closed loop lasercontrol. Laser projection apparatus 500 includes radiance to drivecurrent mapping component 110 and control circuit 150, both of which aredescribed above with reference to FIG. 1. Laser projection apparatus 500also includes digital-to-analog converter (DAC) and driver 516, lasingdevice 518, photodetector (PD) 550, integrator 552, analog-to-digitalconverter (ADC) 554, and accumulator 540.

Radiance to drive current mapping component 110 includesradiance-to-current (R-I) look-up table (LUT) 504, radiance gain block502 to scale commanded radiance by a radiance gain value a, drivecurrent gain block 506 to scale the drive current value by a drivecurrent gain value β, and offset block 512 to offset the drive currentvalue by a drive current offset value γ. Component 110 also includes abias control component that includes delay circuit 508, bias controlcircuit 510, and minimum bias circuit 514.

In operation, a commanded radiance value is provided to the R-I mappingcomponent 110, which provides a drive current value to DAC/driver 5 16.DAC/driver 516 produces an analog drive current that is driven throughlasing device 518. DAC/driver 516 includes a digital-to-analog converteras well as driver circuitry to drive the lasing device. The DAC mayaccept any number of digital input bits and may have any resolution. Forexample, in some embodiments, the DAC may accept eight, ten, or moreinput bits. The driver circuit may include analog circuits such asamplifiers and drivers suitable to drive lasing devices.

Photodetector 550, integrator 552, and ADC 554 form a radiancemeasurement apparatus. Photodetector 550 measures the radiance of lasingdevice 518. Photodetectors typically output a current that is a functionof the measured light. Accordingly, a transimpedance amplifier may beincorporated into PD 550 to convert the current into a voltage.Integrator 552 integrates the measured radiance to provide an average ofthe PD output over an integration period. The integration period may beany length. In some embodiments, the integration period is long enoughto increase the signal-to-noise ratio (SNR) of the PD output, but isshort enough to allow fast closed loop control. In some embodiments,integrator 514 is incorporated into the photodetector by virtue of thenatural response time of the photodetector. For example, the lasingdevice may emit pulsed light on a pixel by pixel basis, and thephotodetector output may reflect the average amount of light incident onthe photodetector over a number of pixels.

ADC 554 converts the output of integrator 552 to a digital value andprovides that digital value to control circuit 150. Control circuit 150iteratively modifies R-I mapping control parameters (α, β, γ) using agradient descent operation as described with reference to FIG. 1.

The commanded radiance is provided to accumulator 540 which sums thecommanded radiance over the same integration period as integrator 552.In some embodiments, integrator 552 is omitted, and accumulator 540integrates over a period to match the natural response of circuits inthe measurement path, such as PD 550.

In some embodiments, the integration period is set such that the R-Imapping control parameters (α, β, γ) are modified fast enough tocompensate for dynamic changes in the behaviour of lasing device 518.For example, the integration period may be set to less than a video linesuch that the R-I mapping control parameters (α, β, γ) are updatedmultiple times over each video line.

In some embodiments, control circuit 150 determines new values for R-Imapping control parameters (α, β, γ) after each integration period andthen slews the actual values of R-I mapping control parameters (α, β, γ)during the next integration period rather than abruptly changing them.For example, if an integration period lasts for 50 pixels, then the R-Imapping control parameters (α, β, γ) may be modified by one fiftieth ofthe entire change value for each pixel.

R-I LUT 504 is a modifiable device capable of storing mappings fromcommanded radiance values to lasing device current values. For example,R-I LUT 504 may be a random access memory device to hold the look-uptable. In some embodiments, the look-up table may be implemented in adual port memory that can be modified from one port and “looked up” fromanother port. In other embodiments, R-I LUT 504 may be replaced by amathematical function to map radiance values to drive current values.R-I LUT 504 may be implemented in any manner without departing from thescope of the present invention.

Bias control circuit 510 “looks ahead” in the video path andconditionally provides a minimum bias value for the drive current value.For example, if the commanded radiance has a very low value for a numberof pixels (e.g., for a video line), then bias control circuit 510 mayprovide a drive current value below a turn-on threshold, but largeenough to preheat laser 518. In some embodiments, bias control circuit510 preheats laser 518 only when future pixels are to be displayed. Forexample if laser 518 turns on at 100 mA of current (threshold of 100mA), and future pixels in the video path call for 90 mA or less, thenbias control circuit 510 does not provide a minimum bias. However, iffuture pixels call for more than 100 mA of current (corresponding tonon-zero future light output), and the current pixels call for less than90 mA, then for the current pixels, and up until the future pixels callfor the non-zero light output, bias control circuit 510 may inject aminimum bias current of 90 mA to preheat laser 518.

Preheating laser 518 may provide a faster turn-on capability. Forexample, some types of lasing devices may turn on very quickly whenpreheated, but may take longer to turn on when not preheated.

Laser projection apparatus 500 shows a single laser 518 with a singleradiance to drive current mapping component 110. In some embodiments,laser projection apparatus 500 is a single color channel in a colorlaser projector. For example, laser projection apparatus 500 may be agreen color channel, and may be combined with a blue and red colorchannel. The blue and red color channels may or may not include acorresponding control circuit 150.

FIG. 6 shows a color laser projection device with closed loop lasercontrol. Laser projection device 600 includes red/green/blue (RGB)radiance-to-current (R-I) mapping components 602, RGB DAC/drivers 606,RGB lasers 608, RGB photodetectors and associated circuits 620, digitalfilters 640, and control circuits 630.

RGB R-I mapping components 602 receive commanded radiances for red,green, and blue lasing devices. In some embodiments, the R-I mappingcomponents 602 are look-up tables that are maintained in separate memorydevices, and in other embodiments, R-I mapping components 602 arelook-up tables share a single memory device. In still furtherembodiments, R-I mapping components 602 are implemented as mathematicalfunctions. The R-I mapping components independently map red, green, andblue commanded radiance values to lasing device drive current values.

RGB DAC/drivers 606 convert the RGB laser drive current values to drivecurrents, and RGB lasing devices 608 are driven with those currents toproduce a composite color pixel. For example, RGB lasing devices 608 maycorrespond to IS lasing devices 302, 304, and 306 (FIGS. 3, 4).

RGB photodetectors and associated circuits 620 detect the amount oflight emitted from laser diodes 608. For example, the RGB photodetectorsmay be photodetectors 340, 350, and 360 (FIG. 3) or photodetector 460(FIG. 4). The associated circuits may include integrators, ADCs,calibration circuits, as well as other suitable circuitry to conditionand produce the RGB measured radiances at 622.

The RGB commanded radiances are also provided to digital filters 640.Digital filters 640 filter the commanded radiance values prior tocomparison with the measured radiance values at 622. In someembodiments, digital filters 640 attempt to compensate for the responseof all circuits in the loop. For example, in some embodiments, digitalfilters 640 compensate for the response of the drivers in RGBDAC/drivers 606 as well as the response of the RBG photodetectors.

Control circuits 630 compute the error between radiance values for eachcolor provided at 622 and 642. In some embodiments, the errorinformation is used to modify the R-I mapping component for one or morecolors to correct for radiance variations. For example, if the measuredradiance of all three lasing devices is differs from the commandedradiance, then parameters (α, β, γ) for each of the R-I mappingcomponents may be modified to effect changes in lasing device drivecurrents. Also for example, if the measured radiance of one lasingdevice is below the expected radiance, then parameters (α, β, γ) for theR-I mapping component corresponding to that lasing device may bemodified to effect a change in the lasing device drive current.

In some embodiments, control circuits modify the R-I mapping componentsfor one or more colors for color balance tracking. For example, the R-Imapping components may be modified to maintain radiance ratios betweenthe colors to maintain a white point, such as a D65 white point. In someembodiments, the R-I mapping component for one color is modified tocorrect for radiance variation, and the remaining two R-I mappingcomponents are modified to correct for color balance variations. Forexample, in some embodiments, the green lasing device may besignificantly more sensitive to temperature variations. In theseembodiments, a look-up table corresponding to the green lasing devicemay be modified to correct for luminance variations, and look-up tablesfor the red and blue lasing devices may be modified to maintain theproper ratios for color balance.

In some embodiments, the red, green, and blue mapping components 602 areupdated at different rates. For example, using different integrationperiods, control circuits 630 may update each of the red, green, andblue mapping components at different rates. For example, in someembodiments, the green lasing device may be more sensitive totemperature variations, and so may be updated more frequently than thered and blue.

The various components shown in FIG. 6 may be implemented in hardware,software, or any combination thereof For example, digital filters 640and/or control circuits 630 may be implemented in dedicated hardwarecircuits, parameterizable hardware circuits, or in software running on aprocessor.

FIG. 7 shows a flowchart in accordance with various embodiments of thepresent invention. In some embodiments, method 700, or portions thereof,is performed by a laser projector, a mobile device, or the like,embodiments of which are shown in previous figures. In otherembodiments, method 700 is performed by an integrated circuit or anelectronic system. Method 700 is not limited by the particular type ofapparatus performing the method. The various actions in method 700 maybe performed in the order presented, or may be performed in a differentorder. Further, in some embodiments, some actions listed in FIG. 7 areomitted from method 700.

Method 700 is shown beginning with block 710 in which commanded radiancevalues used to drive a lasing device are received. This corresponds toR_(C) as shown in the previous figures. The commanded radiance valuesmay be accumulated over a number of pixels in an image (540, FIG. 5), ormay be for single pixels. For example, the commanded radiance values maybe accumulated over an integration period that is less than a scan linein a video image.

At 720, measured radiance values are received. The measured radiancevalues represent light emitted from the lasing device. The measuredradiance values may be integrated over the same integration period asthe commanded radiance values. Appropriate scaling may be applied toboth the commanded radiance values and the measured radiance values suchthat a difference between them is substantially zero when the lasingdevice is performing as desired. If the lasing device is not performingas desired (e.g., the measured radiance differs from the commandedradiance), then a radiance to drive current mapping is modified using agradient descent operation as further described herein.

At 730, at least one partial differential with respect to a firstvariable of a difference between the commanded radiance values and themeasured radiance values is evaluated. This operation corresponds to theevaluation of one or more of eqs. (11)-(13), above, where the firstvariable is any of (α, β, γ). At 740, a new value is determined for thefirst variable using a gradient descent operation. The gradient descentoperation is shown in eqs. (2)-(4), above.

At 750, the first variable is applied to a circuit that performsradiance to drive current mapping. In some embodiments, this correspondsto control circuit 150 applying one of (α, β, γ) to mapping component110 (FIGS. 1, 5). In some embodiments, the first variable is changedabruptly after each iteration of the gradient descent operation, and inother embodiments, the first variable is slewed from one steady statevalue to another between iterations.

The operations of method 700 may be applied to one or more colorchannels in a color laser projector. For example, in some embodiments,apparatus 500 (FIG. 5) may be instantiated three times: once each forred, green, and blue. (See FIGS. 3, 4). In these embodiments, one ormore of (α, β, γ) are modified for each channel. In other embodiments,method 700 may be applied to one color channel in a color laserprojector. For example, a particular green laser embodiment may be moresensitive to temperature effects, and apparatus 500 (FIG. 5) may beincluded for the green color channel, but not the red and blue colorchannel.

FIG. 8 shows a mobile device in accordance with various embodiments ofthe present invention. Mobile device 800 may be a hand held projectiondevice with or without communications ability. For example, in someembodiments, mobile device 800 may be a handheld projector with littleor no other capabilities. Also for example, in some embodiments, mobiledevice 800 may be a portable music player. Also for example, in someembodiments, mobile device 800 may be a device usable forcommunications, including for example, a cellular phone, a smart phone,a personal digital assistant (PDA), a global positioning system (GPS)receiver, or the like. Further, mobile device 800 may be connected to alarger network via a wireless (for example, WiMax) or cellularconnection, or this device can accept data messages or video content viaan unregulated spectrum (for example, WiFi) connection.

Mobile device 800 includes laser projector 801 to create an image withlight 808. Similar to other embodiments of projection systems describedabove, mobile device 800 may include closed loop laser control tocontrol mapping of radiance to drive current values and also to provideminimum drive current values to preheat lasing devices.

In some embodiments, mobile device 800 includes antenna 806 andelectronic component 805. In some embodiments, electronic component 805includes a receiver, and in other embodiments, electronic component 805includes a transceiver. For example, in GPS embodiments, electroniccomponent 805 may be a GPS receiver. In these embodiments, the imagedisplayed by laser projector 801 may be related to the position of themobile device. Also for example, electronic component 805 may be atransceiver suitable for two-way communications. In these embodiments,mobile device 800 may be a cellular telephone, a two-way radio, anetwork interface card (NIC), or the like.

Mobile device 800 also includes memory card slot 804. In someembodiments, a memory card inserted in memory card slot 804 may providea source for video data to be displayed by laser projector 801. Memorycard slot 804 may receive any type of solid state memory device,including for example, Multimedia Memory Cards (MMCs), Memory StickDUOs, secure digital (SD) memory cards, and Smart Media cards. Theforegoing list is meant to be exemplary, and not exhaustive.

Although the present invention has been described in conjunction withcertain embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the scope of theinvention as those skilled in the art readily understand. Suchmodifications and variations are considered to be within the scope ofthe invention and the appended claims.

1. An apparatus comprising: image processing circuitry to command aradiance value; a mapping component to convert the radiance value to adrive current value according to:I=β×f(αR)+γ; wherein I is the drive current, R is the radiance value, ais a radiance gain value, β is a drive current gain value, and γ is adrive current offset value; and a control circuit to use a gradientdescent operation to iteratively determine values for α, β, and γ. 2.The apparatus of claim 1 further comprising: a first lasing devicecoupled to be driven by the drive current; a photodetector to measureradiance of light produced by the lasing device; and an integratorresponsive to the photodetector coupled to integrate measured radianceover less than a video line and provide a result to the control circuit.3. The apparatus of claim 2 further comprising a second lasing deviceand a third lasing device.
 4. The apparatus of claim 3 wherein the firstlasing device comprises a green laser.
 5. The apparatus of claim 1further comprising a minimum bias circuit to look ahead in a video pathand preheat the first lasing device based on future pixel data.
 6. Theapparatus of claim 5 wherein the minimum bias circuit includes a biascontrol circuit to produce a minimum drive current below a turn-onthreshold of the lasing device.
 7. A circuit comprising: a lasing deviceto emit light; a look up table to map commanded radiance values to drivecurrent values for the lasing device; a first gain block to scale thecommanded radiance values by a radiance gain, α, prior to the look uptable; a second gain block to scale the drive current values by a drivecurrent gain, β; and a minimum bias circuit to provide a minimum bias tothe drive current values to preheat the lasing device.
 8. The circuit ofclaim 7 further comprising a control circuit to produce control signalsfor α and β using a gradient descent operation.
 9. The circuit of claim8 further comprising a radiance measurement apparatus to detect lightemitted from the lasing device and to provide a measured radiance R_(M)to the control circuit.
 10. The circuit of claim 8 further comprising anoffset block to offset the drive current values by a drive currentoffset, γ.
 11. The circuit of claim 9 wherein the radiance measurementapparatus includes an analog integrator to integrate the measuredradiance over less than a scan line in a video image.
 12. A methodcomprising: receiving commanded radiance values used to drive a lasingdevice; receiving measured radiance values representing light emittedfrom the lasing device; evaluating at least one partial differentialwith respect to a first variable of a difference between the commandedradiance values and the measured radiance values; determining a newvalue for the first variable using a gradient descent operation; andapplying the first variable to a circuit that performs radiance to drivecurrent mapping.
 13. The method of claim 12 wherein the first variableis a radiance gain value, a, to scale the commanded radiance value. 14.The method of claim 12 wherein the first variable is a drive currentgain value, β, to scale a drive current value produced by the drivecurrent mapping.
 15. The method of claim 12 wherein the first variableis a drive current offset value, γ, to offset the drive current value.16. The method of claim 12 wherein applying the first variable to acontrol circuit comprises slewing the first variable value betweeniterations of determining the new values for the first variable.
 17. Amobile device comprising: a laser projector to project an image, thelaser projector including a lasing device subject to heating based onvideo content, the laser projector further including at least onephotodetector to detect light emitted from the lasing device, the laserprojector further including a mapping component to map desired radiancevalues to a drive current value for the lasing device, and the laserprojector further including a minimum bias circuit to provide a drivecurrent below a turn-on threshold to the lasing device to preheat thelasing device in anticipation of future pixels to be displayed.
 18. Themobile device of claim 17 wherein the minimum bias circuit includes adelay circuit to delay drive current values by less than one video line.19. The mobile device of claim 17 further comprising a memory card slot.20. The mobile device of claim 17 further comprising a communicationsreceiver.